Dual neutron flux/temperature measurement sensor

ABSTRACT

Simultaneous measurement of neutron flux and temperature is provided by a single sensor which includes a phosphor mixture having two principal constituents. The first constituent is a neutron sensitive Li 6 F and the second is a rare-earth activated Y 2 O 3  thermophosphor. The mixture is coated on the end of a fiber optic, while the opposite end of the fiber optic is coupled to a light detector. The detected light scintillations are quantified for neutron flux determination, and the decay is measured for temperature determination.

This application is a continuation of Ser. No. 08/320,411 filed Oct. 3,1994 now abandoned, which is a divisional of Ser. No. 08/225,363 filedApr. 8, 1994 now U.S. Pat. No. 5,352,040, which is a continuation ofSer. No. 07/933,372 filed Aug. 24, 1992 now abandoned.

This invention was made with Government support under contractDE-AC05-840R21400 awarded by the U.S. Department of Energy to MartinMarietta Energy Systems, Inc. and the Government has certain rights inthis invention.

FIELD OF THE INVENTION

The present invention relates generally to radiation interactionmeasurement devices and temperature measurement devices and, morespecifically, to a dual neutron flux/temperature measurement sensorwhich utilizes a phosphor mixture having two principal constituents, onebeing neutron sensitive and the other being temperature sensitive.

BACKGROUND OF THE INVENTION

It is well known that emission properties of phosphors vary inaccordance with temperature. This correlation has been used to devisevarious types of thermometry hardware. For example, surface temperatureof a rotating flywheel has been measured by inducing fluorescence from apulsed nitrogen laser in a material that includes lanthanum oxysulfidedoped with europium. The temperature dependence of the phosphor emissionhas been shown both in amplitude and lifetime changes. With a pulsedlaser as the stimulating source, either the ratio of two emission lineintensities (amplitudes) or the lifetime of some selected line can beused to determine the temperature.

In the field of nuclear reactor engineering, the interactions ofneutrons with nuclei are important to the release of nuclear energy in aform capable of practical utilization. Inelastic neutron collisions donot occur below energies of about 0.1 Mev, but elastic collisionsbetween neutrons and nuclei will be effective in slowing down theneutrons until their average kinetic energy is the same as that of theatoms of a scattering medium. This energy depends on the temperature ofthe medium, and is thus referred to as thermal energy. Neutrons whoseenergies have been reduced to values in this region are designated“thermal neutrons”.

Phosphors have been used to measure thermal neutron flux. A mixture ofboron-containing plastic and ZnS(Ag) phosphor has been used to provide aslow-neutron scintillator. A slow neutron passing through thescintillator is captured by a B10 nucleus. The resultant energetic alphaand lithium particles reach a ZnS(Ag) granule with sufficient residualenergy to cause a scintillation. Light from the scintillation travels tothe photomultiplier photocathode and reaches it with sufficientintensity to cause a recognizable pulse at the anode. The slow-neutronscintillators have been made by using a transparent bioplastic mold castfrom a negative steel mold. In use, the surface of the scintillatorfaces a photomultiplier, while the opposite surface is covered withaluminum foil or other light reflective coating. See, for example, “HighEfficiency Slow-Neutron Scintillation Counters”, NUCLEONICS, by K. H.Sun et al. (July, 1956).

The extreme environment of some nuclear reactor cores, with temperaturesin the range of 1,000° C., presents a difficult problem for sensing bothtemperature and neutron flux. A need exists for an improved sensorcapable of simultaneously measuring both neutron flux and temperature.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a single sensor capableof providing simultaneous measurement of both neutron flux andtemperature.

Another object of the present invention is to provide a sensor which iseasy to install and relatively simple in construction.

These and other objects of the invention are met by providing a dualneutron flux/temperature measurement sensor which includes a phosphormixture having a first neutron-sensitive phosphor constituent and asecond activated thermophosphor constituent coated on an end of a fiberoptic, and means for detecting light generated by charged particlesproduced by neutron absorption in the first constituent. The firstconstituent produces the charged particles when neutrons are absorbedtherein, and the charged particles produce scintillations in the secondconstituent. The scintillations of the second constituent are detectedand correlated to a temperature value which varies in accordance withvariations in the detected scintillations. The second constituent ispreferably a rare-earth activated thermophosphor.

Other objects, advantages and salient features of the invention willbecome apparent from the following detailed description, which, taken inconjunction with the annexed drawings, discloses preferred embodimentsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a dual neutron flux/temperature measuringsensor according to a preferred embodiment of the present invention;

FIG. 2 is a graph showing europium-activated yttrium oxide showing thelogarithmic dependence of the fluorescence decay rate on temperature;and

FIG. 3 is an illustration of the peak emission spectrum's amplitudedependence of europium-activated yttrium oxysulfide on increasingtemperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1, 2 and 3, a dual neutron/flux temperaturemeasurement sensor 10 includes a coating 12 made of a phosphor mixturehomogeneously distributed within an optically transparent binder. Themixture is applied to and forms the coating 12 on the bare tip of afiber optic 14. The mixture includes a first neutron sensitive phosphorconstituent and a second activated thermophosphor constituent. Thesecond constituent is preferably a rare-earth activated thermophosphor,but may also be a metal activated thermophosphor. When the sensor 10 isused to sense conditions in a nuclear reactor, thermal neutrons aredetected in the first phosphor constituent via neutron absorption. Whenthe neutrons are absorbed within the first constituent, chargedparticles are created which in turn produce scintillations in theactivated thermophosphor. The ambient temperature surrounding thecoating can be monitored by observing the ratio of two emission lineamplitudes and/or the decay rates of the scintillations from theactivated thermophosphor.

Neutrons are neutral particles which normally are detected throughnuclear reactions which result in energetic charged particles such asprotons, alpha particles, etc. Conventional methods can then beincorporated to detect the charged particles. According to the presentinvention, a scintillation technique is preferable for detecting thecharged particles created from the absorptions of a neutron.

In one particular embodiment, the mixture forming the coating 12includes as the first constituent Li⁶F (95% Li⁶)and rare-earth activatedY₂O₃. The Li⁶F (95% Li⁶)has a high cross-section for thermal neutrons(940 barns) and, when the neutron is absorbed, produces an alpha and atriton. These charged particles are then detected by the rare-earthactivated Y203, producing visible light scintillations which are guidedto a light collector 16 by the fiber optic 14. The light collector 16can be a photodiode or a photomultiplier tube, for example.

The fiber optic may be made of quartz or sapphire or other comparablematerials that are transparent to the scintillation light. The thicknessof the phosphor material coated onto the tip of the fiber is such thatthe light pulses leaving the coating 12 are not significantlyattenuated.

Thermographic phosphors have a useful property in that the luminescenceof the phosphor changes in emission line amplitude and decay rate withchanges in temperature. As shown in FIG. 2, as the temperature of thesurrounding environment is increased, the lifetime of the fluorescenceinduced in the phosphor decreases logarithmically. The graph showslifetime verses temperature for europium-doped yttrium oxide. When theneutron is absorbed in the Li⁶F, charged particles are produced whichcreate scintillations in the rate-earth activated Y₂O₃. The lifetime andemission line amplitude of the scintillations will be determined by thecharacteristic properties of the rare-earth activated Y₂O₃, mainly thetemperature of the phosphor. If the temperature of the sensorsurroundings changes, this will be indicated by a change in the ratio oftwo emission line amplitudes and decay rate of the inducedscintillations which are detected by the light collector 16 at the endof the fiber optic 14 opposite the phosphor coated end.

Light from the fiber optic 14 passes through a bandpass filter 18 beforeentering the light collector 16. Once the scintillations reach the lightcollector 16, the pulses are amplified. Amplification preserves the timeemission peak characteristics of the pulse which is simultaneouslydirected to a discriminator-counter 20 for determining neutron flux anda waveform digitizer 22 or other suitable device to obtain thetemperature dependent pulse decay constant or ratio of two emission lineamplitudes. Other suitable means may be employed for performing thefunctions of the discriminator-counter 20 and the waveform digitizer 22.

A practical use of the sensor 10 which incorporates a Li⁶F phosphormixture is to measure tritium production at a point in a reactor or zeropower experiment and also simultaneously measure temperature.

Many alternative coatings can also be made which serve as both a neutronabsorber and scintillator. For example, Y₂O₃:Gd could be used as theneutron-sensitive activated thermophosphor. Using a singleneutron-sensitive thermophosphor has advantages over mixing aneutron-sensitive phosphor and a thermophosphor, in that there is noconcern over optimizing the ratio of the two phosphors. Moreover, thereis no potential for inhomogeneity due to inadequate mixing.Advantageously, the large cross-section for gadolinium allows forthinner phosphor layers, thus reducing any gamma interactions.

Measurements of other radiation interactions can be achieved byselecting a radiation-sensitive material in the phosphor mixture to becompatible with the type of interaction being measured, such asmeasuring fission fragments, beta particles, etc., with alternativeversions used at reactors, fusion machines, or accelerators.

The binder material can be of any suitable material which is opticallytransparent. Binders also exhibiting radiation resistance could be used,and would provide for measurements in high radiation fields. As anexample, a colorless polyester can be used as the binder material.

The sensitivity of the sensor 10 can be adjusted by varying the amountof reacting material in the coating 12. This feature may have particularsignificance where the sensor 10 is required not to significantlyattenuate the radiation beam or production. Also, the nuclear reactingconstituent mixed with the thermophosphor constituent can be varied toutilize reaction rates for other material while simultaneously measuringthe temperature.

If a high-temperature thermophosphor is selected, the temperature can bemonitored in environments up to 1500° C., depending on thethermophosphor used, the survivability of the binder/fiber, and on thetemperature limit of the thermophosphor. Therefore, the probe can becustomized to specific temperature ranges by choosing appropriatethermophosphors.

The electronic components for processing the signal output from thelight detector 16 are conventional. Each of the discriminator-counter 20and the waveform digitizer can be provided with visual displays 24 and26, respectively, indicating the respective measured values of neutronflux and temperature. A commercially available waveform digitizersuitable for use in the present invention is sold by Tektronix as modelno. 7854. For a general description of similar components, see R.Stedman, “Scintillator for Thermal Neutrons Using Li⁶F and ZnS(Ag), Rev.Sci. Instrum., 31, 1156, and K. H. Sun et al., “High-efficiencySlow-neutron Scintillation Counters”, Nucleonics, 14(7), 46(1956), bothof which are incorporated herein by reference.

While advantageous embodiments have been chosen to illustrate theinvention, it will be understood by those skilled in the art thatvarious changes and modifications can be made therein without departingfrom the scope of the invention as defined in the appended claims.

What is claimed is:
 1. A scintillator comprising: a firstneutron-sensitive phosphor constituent and a second thermophosphorconstituent, the first and second constituents being homogeneouslydistributed in an optically transparent binder, the first phosphorconverting neutron particles to charged alpha particles by neutronabsorption when exposed to radiation interaction, and the secondthermophosphor converting the charged alpha particles to scintillationswhich vary in accordance with variations in ambient temperaturesurrounding the scintillator, wherein the first constituent is Li⁶F andthe second constituent is rare-earth activated Y₂O₃.
 2. The scintillatoraccording to claim 1, wherein the Li⁶F is 95% Li⁶F.
 3. A scintillatorcomprising: a first neutron-sensitive phosphor constituent and a secondthermophosphor constituent, the first and second constituents beinghomogeneously distributed in an optically transparent binder, the firstphosphor converting neutron particles to charged alpha particles byneutron absorption when exposed to radiation interaction, and the secondthermophosphor converting the charged alpha particles to scintillationswhich vary in accordance with variations in ambient temperaturesurrounding the scintillator, wherein the second constituent includeseuropium.
 4. A scintillator comprising: a neutron-sensitive activatedthermophosphor constituent homogeneously distributed in an opticallytransparent binder, and converting neutron particles to charged alphaparticles by neutron absorption when exposed to radiation interaction,the thermophosphor constituent converting the charged alpha particles toscintillations which have a pulse decay time which varies in accordancewith ambient temperature surrounding the scintillator, wherein theneutron-sensitive, activated thermophosphor constituent isneutron-sensitive, activated Y₂O₃.
 5. A scintillator comprising: aneutron-sensitive activated thermophosphor constituent homogeneouslydistributed in an optically transparent binder, and converting neutronparticles to charged alpha particles by neutron absorption when exposedto radiation interaction, the thermophosphor constituent converting thecharged alpha particles to scintillations which have a pulse decay timewhich varies in accordance with ambient temperature surrounding thescintillator, wherein the neutron-sensitive activated thermophosphor isactivated by gadolinium.